7 replies to this topic

[Faisal Ghafoor]

I have seen in some ammonia plant that condenser at ammonia refrigeration compressor discharge is elevated; almost 10 m from the ground level. Can anybody guide what is the purpose and what if we don't elevate?

[fallah]

Hi,

 

What is the elevation of compressor itself from the ground level?

[Faisal Ghafoor]

Not sure; but looking more or less same. Will check design documents? Is there any relation w.r.t backflow from receiver to compressor?

 

If so, why design with condenser at ground level is also existing.

[fallah]

Condenser could be at an elevation lower than that of the compressor discharge line to avoid back flow toward the condenser ...Subsequent receiver elevation will dictate the exact elevation of the condenser...It might in the case of condenser at ground level the liquid will be pumped to the receiver..

[Faisal Ghafoor]

Receiver elevation is 7 ft and condenser is 37 ft. In the plant where condenser is at ground level, there is no pump for transferring liquid from condenser to receiver. The condenser at ground level is probably designed for more liquid level built up and more subcooling.

[Art Montemayor]

Faisal:

 

The reason a refrigerant condenser is normally located at an elevation is to facilitate the automatic, gravity-driven drainage of saturated liquid condensate from the condenser to the receiver where it is stored for subsequent use downstream in the system’s evaporator.  It is practical common sense to employ gravity wherever possible to avoid the use of pumps, more piping, and control instrumentation.  All this extra, un-needed equipment provokes refrigerant leaks and maintenance - plus un-needed capital expense.

 

Why a condenser would be located at an elevation of 37 feet is anybody’s guess within our Forum.  Only local conditions, needs, or even erroneous decisions can answer that query.  Maybe someone had a wild idea that the air-cooled condenser would work better at a higher elevation (I guess that it is air-cooled; you haven’t told us).  The same answer applies to placing a refrigerant condenser at ground level.  Even if you were using a pumped-around refrigerant circuit, an elevated condenser and receiver would make sense for a positive NPSH on the circulating pump.  I have never - in 55 years - even heard of any refrigerant condenser being placed at ground level …. But then again, I’ve seen and witnessed worse things done by some engineers.

 

I offer the following learning experience:  more liquid level built up DOES NOT MEAN more liquid refrigerant subcooling in the receiver.  In fact, it is practically impossible to obtain any subcooling in a static liquid within its closed system.  The only way I’ve sub-cooled (and here, I use the term as defined in an engineering manner: a liquid cooled below its saturated temperature while existing at its corresponding saturated pressure) a refrigerant is while it is flowing (usually prior to its expansion valve) and under kinetic pressure (NOT STATIC) from the receiver.  All refrigerant in its receiver is saturated; it HAS TO BE because it is in equilibrium with its vapor pressure - which is set by the condensing temperature.  Therefore, your comment about subcooling the receiver liquid is erroneous.  If you cool the receiver liquid below the ambient condensing temperature while it remains in the receiver, it will simply assume the corresponding vapor pressure - which is lower than the vapor pressure it has at ambient conditions.

[Bobby Strain]

Could be the receiver is full and surge is elsewhere. Then the elevated condenser makes sense. This would deliver liquid refrigerant to control valves in a sub-cooled state. So the valves would not require larger sizes to accommodate the vapor associated with saturated liquid from the source. So maybe you will enlighten us as to what your whole system is. Not just a peek at one small portion. It's like the forrest and the trees.

 

Bobby

[Faisal Ghafoor]

Ours is a cooling water condenser at an elevation of 37 ft. Receiver elevatioin is 20ft. This is part of closed loop refrigeration system with ammonia compressor of 6 MW installed in ammonia plant back-end.

 

I appreciate the logical explanation by Art Montemayor. I didn't  mean more level build up in condenser will increase sub-cooling. I have expereince of operating condenser with almost 20% level (no level measurement, but temperature difference tells the level). Whenever liquid level profile changed in the condenser, variation in pressure observed. As rightly told, the higher level will reduce the available surface for condensation.

I remember I read somewhere that sllght sub cooling (~5F equivalent) is kept in design to take care of pressure drop from condenser to receiver (nozzles and gravity drain pipe). I don't have the exact reference of this.Eventually the liquid in receiver will be at exact saturation temperature, while in condenser outlet  it would be slightly subcooled.

I am interested in knowing in designing how much elevation should I consider to allow free draining; will any number like 5 ft , 10 ft or 20 ft will be okay or there will be some criteria?